Disk drives are commonly used in workstations, personal computers, laptops and other computer systems to store large amounts of data in a form that can be made readily available to a user. In general, a disk drive comprises a magnetic disk that is rotated by a spindle motor. The surface of the disk is divided-into a series of data tracks that extend circumferentially around the disk. Each data track stores data in the form of magnetic transitions on the disk surface.
An interactive element, such as a magnetic transducer, is used to sense the magnetic transitions to read data, or to transmit an electric signal that causes a magnetic transition on the disk surface, to write data. Typically, the magnetic transducer is mounted in a head. The head, in turn, is mounted by a rotary actuator arm and is selectively positioned by the actuator arm over a preselected data track of the disk so that the transducer can either read data from or write data to the preselected data track of the disk, as the disk rotates below the transducer.
A gap is provided in the head to position the active elements of the transducer at a position suitable for interaction with the magnetic transitions. In certain modern transducer structures, dual gaps are provided in the head, one for positioning a read transducer and the other for positioning a write transducer. In this manner, separate technologies can be used for each of read and write transduction operations to enhance the overall effectiveness of a disk drive product. Moreover, the use of separate gaps accommodates design constraints that would preclude mounting different transduction technologies in the same physical gap.
For example, in a magnetoresistive head (MR head), a magnetoresistive element (MR element) is used as a read transducer. The magnetoresistive element comprises a material that exhibits a change in electrical resistance as a function of a change in magnetic flux of a magnetic field applied to the element. In a disk drive environment, the MR element is positioned within the read transducer gap, above a disk surface. In this position, the electrical resistance of the element changes in time as magnetic transitions recorded on the disk pass beneath the read gap, due to rotation of the disk. The changes in the resistance of the MR element caused by magnetic transitions on a disk occur far more quickly than the response of conventional transducers to magnetic transitions. Thus, an MR transducer is able to sense magnetic transitions during a read operation at higher rotational speeds and data densities.
However, an MR element is not able to transmit a signal in a manner that efficiently generates a magnetic field, which is necessary to write data on a disk surface. Accordingly, an inductive circuit is used as the write transducer and is positioned in the separate write gap.
Whenever data is either written to or read from a data track, the appropriate transducer gap is centered by the actuator arm over the centerline of the data track where the data is to be written or from where the data is to be read, to assure accurate transduction of the data. Thus, in a read operation, the gap associated with the MR element is centered over the appropriate data track centerline and the gap associated with the inductive write circuit will be offset from that track centerline due to a skew angle effect between the gaps. The opposite is true in a write operation.
An important aspect of conventional disk drive design relates to position control of the heads. The position control is used to accurately position a head over a data track for data read or write operations. As noted above, whenever data are either written to or read from a particular data track, the appropriate transducer gap of the corresponding head is preferably centered over the centerline of the magnetic transitions of the data track where the data are to be written or from where the data are to be read, to assure accurate transduction of the transitions representing data. If the head is off-center, the head may transduce transitions from an adjacent track.
A servo system is typically used to control the position of the actuator arm to insure that the head is properly positioned over the magnetic transitions during either a read or write operation. In a known servo system, servo position information is recorded on the disk surface itself, and periodically read by the head for use in a track following operation to control the position of the actuator arm. Such a servo arrangement is referred to as an embedded servo system. In modern disk drive architectures utilizing an embedded servo, each data track is divided into a number of data sectors for storing fixed sized data blocks, one per sector. In addition, associated with the data sectors are a series of servo sectors that are generally equally spaced around the circumference of the data track. The servo sectors can be arranged between data sectors or arranged independently of the data sectors such that the servo sectors split data fields of the data sectors, as is well known.
Each servo sector contains magnetic transitions that are arranged relative to a track centerline such that signals derived from the transitions can be used to determine head position. For example, the servo information can comprise two separate bursts of magnetic transitions, one recorded on one side of the track centerline and the other recorded on the opposite side of the track centerline. Whenever a head is over a servo sector, the head reads each of the servo bursts and the signals resulting from the transduction of the bursts are transmitted to, e.g., a control device such as a microprocessor within the disk drive for processing.
When the head is properly positioned over a track centerline, the head will straddle the two bursts, and the strength of the combined signals transduced from the burst on one side of the track centerline will equal the strength of the combined signals transduced from the burst on the other side of the track centerline. The microprocessor can be used to perform the track following operation. The track following operation basically entails the subtraction of one burst value from the other each time a servo sector is read by the head. When the result is zero, the microprocessor will know that the two signals are equal, indicating that the head is properly positioned.
If the result is other than zero, then one signal is stronger than the other, indicating that the head is displaced from the track centerline and overlying one of the bursts more than the other. The magnitude and sign of the subtraction result can be used by the microprocessor to determine the direction and distance the head is displaced from the track centerline, and generate a control signal to move the actuator back towards the centerline.
A write inhibit signal is used to terminate transmission of a write signal to a head during a time period that the servo signal indicates that the head is off center by an amount that would result in writing data in an adjacent track. In conventional disk drives, the threshold for asserting a write inhibit signal is set at the same level for all heads in the disk drive and at a level that is sufficient to accommodate the widest possible write gap width, relative to the width tolerances for the head. This is, in effect, a worst case solution. If the servo system indicates that the head is off track center by an amount that would cause a head, having a widest possible write gap,-to overwrite an adjacent data track, the write inhibit signal is asserted. This is true even if the actual width of the particular head is less than the maximum width, and would not, in fact, overwrite an adjacent data track. Indeed, this is true even if the write gap width is at the minimum end of the tolerance range. Accordingly, the write inhibit signal can be asserted more times than necessary to avoid overwriting adjacent data tracks, resulting in an undue disruption of disk drive operation.
In a dual gap head, the width dimension of the write transducer is typically approximately equal to the radial width of the data track, while the width dimension of the MR transducer is significantly narrower than the width of the write transducer. Advantage can be taken of the relatively narrow MR transducer in connection with servo position control relating to write operations. More specifically, the narrower width of the MR transducer permits greater leeway in setting the level of a write inhibit threshold. A properly positioned MR transducer will be well within the radial width of a data track when used in a read operation and will not read any transitions at the outer portions of the data track. Thus, a head positioned to write data to a particular data track can be displaced from that track centerline, even to the extent that it overlies an adjacent data track, if the amount the head overlies the adjacent data track is beyond the effective read width of the corresponding MR head when that head is properly positioned to read data from the adjacent data track.
However, the write inhibit threshold is still set relative to the widest possible write gap width and widest possible read gap width, and the same threshold is used for all of the heads of the disk drive. Accordingly, despite the added leeway available in disk drives utilizing MR heads, the number of write inhibit signal assertions will still generally exceed the number of assertions actually necessary to protect the integrity of data written onto data tracks.